Electronic devices such as a notebook type computer, a portable phone, and a digital camera are widely used, and thus demands for a secondary battery which is used for driving these electronic devices are increased. Recently, in the electronic devices, consumed power is increased or reduction in size is expected as high functionality progresses. Thus, improvement of performance of the secondary battery is required. Among secondary batteries, in a case of a non-aqueous electrolyte secondary battery (in particular, lithium ion secondary battery), increasing capacity is possible. Thus, the battery has been used in various electronic devices.
Generally, the non-aqueous electrolyte secondary battery has a configuration in which a positive electrode and a negative electrode are connected to each other through a non-aqueous electrolyte (non-aqueous electrolyte liquid) and are stored in a battery case. In the positive electrode, a positive electrode active material layer containing a positive electrode material which is represented as a positive electrode active material is formed on the surface of a positive electrode current collector. In the negative electrode, a negative electrode active material layer having a negative electrode active material is formed on the surface of a negative electrode current collector.
In a lithium ion secondary battery which is the representative example of the non-aqueous electrolyte secondary battery, composite oxide of lithium is used as a positive electrode material (positive electrode active material). Examples of the composite oxide are disclosed in Patent Literatures 1 to 6.
Patent Literature 1 discloses a positive electrode active material obtained by mixing LixCoMO2 and LiNiMnMO2 (M: predetermined element which has been selected). The positive electrode active material includes an active material having a high average voltage during discharging, and an active material having high thermal stability.
Patent Literature 2 discloses a positive electrode active material including a crystal layer which has a layered rock salt type structure of LiNiMnTiO2. The positive electrode active material contains Ti, and thus has charging and discharging capacity higher than that in a case of not being contained.
Patent Literature 3 discloses a positive electrode active material obtained by mixing LixMnMO4 and LiNiMO2 (M: predetermined element which has been selected). The positive electrode active material is excellent in battery performance after preservation at a high temperature.
Patent Literature 4 discloses a positive electrode active material having a layer shape in which a portion of Li in LiMnMO2 having a polycrystalline structure is lacked (M: predetermined element which has been selected). The positive electrode active material causes distortion in crystal or stabilization of chemical bonds to occur. Thus, effects of cycle stability at a time of charging and discharging, durability stability, and the like are obtained.
Patent Literature 5 discloses a positive electrode active material obtained by respectively substituting portions of Li and Co in LiCoO2 with predetermined elements M (M: predetermined element which has been selected). In the positive electrode active material, Li and Co are substituted with elements M, and thus a bonding force between a lithium layer and a cobalt layer is reinforced, and an occurrence of distortion between layers or expansion of a crystal lattice is suppressed. Thus, effects of cycle stability at a time of charging and discharging, durability stability, and the like are obtained.
Patent Literature 6 discloses a positive electrode active material obtained by mixing LiNiMnCoO2 and Li2MO3 (M: predetermined element which has been selected). The positive electrode active material includes an active material which exhibits an effect of excellent battery capacity and safety, and an active material which exhibits an effect of cycle characteristics and storage characteristics.
However, both of the above positive electrode active materials (positive electrode material) have a problem in that sufficiently suppressing collapse of a crystal structure at a time of charging and discharging is not possible, and reducing capacity of a non-aqueous electrolyte secondary battery is caused.
Non Patent Literature 1 discloses a technology of using a positive electrode containing Ti, that is, LiNiMnTiO2 for safety.
However, a situation in which significant improvement of safety does not occur in a case of adding Ti at about 30% as disclosed in Non Patent Literature 1 occurs.
As another attempt for achieving both of safety and high-stability of crystal, Non Patent Literature 2 discloses a technology of using a positive electrode which contains Si having a high bonding force with oxygen, along with transition metal in the same quantity, that is, using Li2MnSiO4.
However, since transition metal in the positive electrode has a four-coordinated coordination structure, the structure becomes unstable during charging. Thus, the above positive electrode is also not a positive electrode having sufficient durability.
Patent Literature 7 discloses a positive electrode active material containing Li oxide represented by Li[LixMeyM′z]O2+d (x+y+z=1, 0<x<0.33, 0.05≤y≤0.15, 0<d≤0.1, Me: at least one type selected from Mn, V, Cr, Fe, Co, Ni, Al, and B, and M′: at least one type selected from Ge, Ru, Sn, Ti, Nb, and Pt).
However, in a battery using this positive electrode active material, improvement of safety is insufficient. Specifically, the addition percentage of an element which is provided in transition metal and is indicated by Me is about 14 at %, and an oxygen atom which is not bound to the element indicated by Me exists. The element indicated by Me and an oxygen atom are strongly bound to each other, and thus breakup (separation of oxygen) of the bonding hardly occurs. That is, oxygen atoms which are included in the positive electrode active material in Patent Literature 7 and are not bound to the element indicated by Me are provided in a form of an oxygen gas, when a battery is formed. Thus, safety of the battery is degraded.
A lithium ion secondary battery (non-aqueous electrolyte secondary battery) has a concern that long-term uses thereof causes a crystal structure in composite oxide of lithium used in a positive electrode active material to be collapsed, and thus contained oxygen is released. In addition, the positive electrode active material has a problem in that the temperature at which oxygen is released is low (breakup of the crystal structure easily occurs if the temperature of the positive electrode active material is increased), and as a result, the safety is degraded.